The present invention relates to a hearing aid test apparatus and, in particular, to a measuring device for checking the tightness or fit of an ear impression mold or of an otoplastic which is insertable into the auditory passage of an ear.
To compensate for hearing loss, persons hard of hearing have been using many varied kinds of hearing aids. Regardless of the specific design of the hearing aids, they all have in common the feature that sound must always be conducted into the ear when hearing rectification is by air conduction as is usually the case. Towards this end a so called otoplastic is inserted into the auditory passage which is completely adapted to the contour of the auditory passage and should fit in a sealing fashion. The sound is conducted from the hearing aid to the ear drum through an air duct in the otoplastic, and it must have no opportunity of reaching the outside through leaky points. For, if the seal is not complete, it is very easy for an acoustic feedback to occur which results in that sound escaping through the leak reaches the hearing aid microphone where it is amplified and reenters the air duct to the ear drum and again escapes to the outside to the microphone thereby generating oscillations audible as a shrill, disagreeable noise. Moreover, if the otoplastic does not fit the auditory passage exactly, the leaky points lead to distortions in the frequency response of the device.
The above discussion indicates that the manufacturing of the otoplastic is of decisive importance. Usually, the so called "positive-negative-positive method" is employed, in which an ear impression is made first, using a compound which is placed into the auditory passage. The compound solidifies after a short while, whereupon a negative can be produced from the impression (positive). For this purpose the impression is embedded in plaster of Paris or plastic material and taken out of it again after the material in which it is embedded has hardened. There now remains a mold corresponding to the ear impression which can now be filled with plastic in various ways. A copy of the ear impression true to the original, i.e. the otoplastic, is obtained in this manner, and if the manufacturing method was precise, it will fit the auditory passage exactly without leaving leaky points. Lastly, a hole for a sound tube is provided in the otoplastic so that the sound can be conducted through this tube from the hearing aid to the ear drum.
Unfortunately, the manufacture of the otoplastic does not always proceed without problems, and it happens that the finished otoplastic often does not fit the auditory passage exactly, thus not assuring the required complete seal. Two factors contribute to the fitting problem of the hearing aid. On the one hand, the ear impression mold may be inexact, e.g. due to the inclusion of air bubbles or due to the shrinkage of the impression material, and on the other hand errors can be made during the manufacture of the otoplastic itself, even if the ear impression serving as a pattern is correct. In the past there were no means of finding the actual source of error when the otoplastic did not seal properly. While checking of the ear impression mold was not possible at all, checking the otoplastic was restricted to a subjective check by questioning the hearing impaired person and examining the device for fit and feedback. If the otoplastic was found defective, the only thing left to do was to repeat the manufacturing process in the hope that the new one will fit the auditory passage exactly.
In German patent application DE-OS No. 2 941 817.5 is described a test method which makes it possible to determine the excellence and quality of an ear impression or of an otoplastic. This method provides for the generation, in the inner space defined by the ear drum and the ear impression or otoplastic, of an air pressure deviating from the air pressure prevailing on the outside via an air permeable connection from the inner space of the ear to the outside. A test is then made to determine whether the pressure remains stable or decreases, the rate at which the pressure changes being determinable by a measuring instrument, giving direct information on the quality and usability of the ear impression.
However, actual construction of such a measuring instrument encounters difficulties because only a very slight pressure of a maximum of about 20 mbar may be applied to the inner ear which has a volume of only about 2 cm.sup.3, a small value which renders the task difficult. If a very slight overpressure is applied directly to the inner ear space, variations from the average volume of the auditory passage, which naturally is not the same in all people, will occur and lead to erroneous test results. The measuring inaccuracies are particularly great in the case of surgically enlarged auditory passages i.e. "radikalhoehlen". Also, the slight overpressure in the small volume fades very quickly even if the leaks are minor, thus rendering a reliable measurement more difficult.
Accordingly, the invention relates to a measuring device to test the tightness of an ear impression serving as pattern for the production of an otoplastic and/or an otoplastic inserted in the auditory passage of an ear, by means of which an air pressure deviating from the air passage prevailing on the outside can be produced, via an air permeable connection, in the inner space defined by the ear drum and the ear impression or the otoplastic, and by means of which changes in the generated deviating air pressure can be measured.